The system architecture evolution (SAE) network is a new generation of the wireless network technology which is established by the 3rd Generation Partnership Project (3GPP) organization and used for replacing the traditional circuit exchanging network, and it includes the Long Term Evolution (LTE) network and the Evolved Packet Core (EPC) network. The framework is shown in FIG. 1, and the function of each network element is described as follows.
The User Equipment (abbreviated as UE) has the ability of accessing two kinds of wireless networks, i.e. the Universal Terrestrial Radio Access Network/Global system for Mobile Communications (GSM) Enhanced Data rates for GSM Evolution (EDGE) Radio Access Network (UTRAN/GERAN) (referring to the General Packet Radio Service (GPRS) radio network, wherein, the UTRAN represents the 3G network, and the GERAN represents the 2G network) and the LTE wireless network. The UE can only reside in one wireless system at one moment (called single radio) because of the ability limitation and the energy conservation requirement, for example, when the UE resides in the LTE, it is unable to receive the paging message from the UTRAN/GERAN at this moment.
The evolved NodeB (abbreviated as eNodeB) can provide higher uplink and downlink speed, lower transmission delay and more reliable wireless transmission than the UTRAN on the air interface. The eNodeB provides the wireless resources for the terminal access, and at the same time, establishes the S1 control plane link with the Mobility Management Entity (MME) of the core network.
The Mobility Management Entity (abbreviated as MME) is a control plane entity, is a server for storing the subscriber data temporarily, and is responsible for managing and storing the UE context (such as, the UE/user identification, the mobility management state, and the user security parameter, etc.) and distributing the temporary identification, such as the Globally Unique Temporary Identity (GUTI) for the user, and the MME is responsible for performing the authentication on the user when the user accesses through the LTE network.
The System Architecture Evolution (SAE) Gateway (GW) (abbreviated as SAE GW) is a user plane entity, responsible for the route processing of the user plane data. The SAE GWs are generally divided into serving gateways (Serving GWs) and Packet Data Network (PDN) GWs, the Serving GW is responsible for the mobility anchor point between the LTE and the Universal Mobile Telecommunications System (UMTS), and in an idle state, the downlink data trigger the MME and the Serving GPRS Support Node (SGSN) paging; the PDN GW is responsible for the gateway function that the UE accesses the Packet Data Network (PDN), which assigns the user Internet Protocol (IP) address for the user. The PDN GW and the Serving GW may be set in one physical entity together.
The Home Subscriber Server (HSS) is used for storing the subscription data of the user, and recording the name of the MME to which the user belongs at the same time.
FIG. 1 provides the framework of the traditional circuit domain switching network at the same time. Wherein, the network element of the core network has a Mobile Switching Center (abbreviated as MSC) and the Visitor Location Register (abbreviated as VLR), and the MSC and VLR are generally combined together physically.
The 3G Radio Network Controller (abbreviated as RNC) and/or the Base Station Controller (abbreviated as BSC) are responsible for controlling and managing the wireless activity of the user.
The traditional circuit domain switching network uses the Home Location Register (abbreviated as HLR) to store the subscription information of the user and the address of the VLR. In the calling process of the called, the HLR can obtain the Mobile Station Roaming Number (abbreviated as MSRN) of the user from the VLR. In FIG. 1, the HLR and the HSS are held by one physical network element.
The Gateway Mobile Switching Center (abbreviated as GMSC) is used for routing the call to the MSC to which the user belongs. When there is a call to the mobile user, the GMSC obtains the MSRN of the user through HLR, and then routes the call according to the MSRN.
The MME element of the SAE network and the VLR of the traditional circuit domain switching network are connected through the SGs interface (that is, the interface between the MME and the MSC).
It is a long-term trend that the SAE network replaces the traditional circuit domain switching network. In this process, because the wireless coverage performance of the traditional circuit domain switching network is better, and the service is comparatively steady, it is the effective supplement for the SAE network. When the SAE network is unable to provide the satisfied traditional speech service, then it is to turn to the traditional circuit domain switching network for providing the relative service. This kind of mechanism of returning to the traditional circuit domain switching network is named the Circuit Switch Fallback (abbreviated as CSFB) technology, and the UE that can fall back to the traditional circuit domain switching network to perform the service is called a Dual Mode Single Radio UE. The CSFB technology establishes the SGs interface between the MME of the SAE network and the VLR of the traditional circuit domain switching network. Through that interface, the MME enables the UE roaming in the LTE to trigger the location update procedure in the traditional circuit domain switching network, thus enabling the UE to register with the traditional circuit domain switching network when the UE is active in the SAE network. As shown in FIG. 2, the process includes the following steps.
In step 201, when the UE in the LTE starts up or changes the Tracking Area (abbreviated as TA), it initiates a startup (ATTACH) or Tracking Area Update (abbreviated as TAU) operation to the MME.
In step 202, the MME receives the above-mentioned message, and performs the registration from the SAE network to the HSS.
In step 203, after the registration is successful, the MME derives out the Location Area Identity (abbreviated as LAI) of the UE in the traditional circuit domain switching network according to the tracking area to which the UE belongs, and it is positioned to the corresponding VLR. Through the SGs interface, the MME sends the location update request to the VLR.
In step 204, if the VLR does not have the subscriber data, or the VLR does not think that the subscriber data are reliable, then the VLR sends the location update request to the HLR.
In step 205, the HLR will send the Insert Subscriber Data (ISD) request to the VLR, and the VLR records the subscription data of the user, and returns the response.
In step 206, the HLR returns a location update success response, and records the number of the VLR to which the user belongs.
In step 207, the VLR returns a location update completion message to the MME, and changes the state of the SGs interface as activated.
The user can receive the call of the called from the traditional circuit domain switching network by way of the CSFB after registering with the HLR, and the process refers to FIG. 3, specifically including the following steps.
In step 301, after receiving the Initial Address Message (IAM) of the outer net, the GMSC finds the called HLR according to the called Mobile Station Integrated Services Digital Network (MSISDN) number (that is, the user telephone number), and sends Send Routing Information (SRI) request to the HLR, with the MSISDN number of the user being carried therein.
In step 302, the HLR finds the MSC/VLR registered by the user according to the MSISDN number, and sends the Provide Roaming Number (PRN) request to the MSC/VLR, and the MSC/VLR is the MSC/VLR registered by the UE from the LTE/EPC network at this moment. The MSC/VLR distributes the temporary user identification, that is, the MSRN, and returns it to the HLR. The MSRN identifies the MSC/VLR and the called UE uniquely.
In step 303, the HLR returns an SRI response to the GMSC, carrying the MSRN.
In step 304, then the GMSC routes the call to the MSC/VLR according to the MSRN.
In step 305, the MSC/VLR initiates the CS paging process from the SGs to the MME, then the MME initiates the CS paging process inside the LTE network, and returns the service request to the MSC/VLR.
In step 306, the UE returns to the circuit domain, and initiates the paging response from the traditional circuit domain switching network.
In step 307, if required, the MSC/VLR performs the access operation.
In step 308, the MSC/VLR sends the call setup request Setup to the UE.
In step 309, the MSC/VLR and the UE finish the left called setup process.
In the above-mentioned process, if the MSC/VLR in the traditional circuit domain switching network breaks down and loses the subscriber data, then the subscriber data can be recovered, including the MME name from the HLR, so as to page. As shown in FIG. 4, the process includes the following steps.
In step 401, the HLR sends the PRN request to the MSC/VLR.
In step 402, the MSC/VLR receives the PRN request, and finds that it does not have the subscriber data, so it sends the Recover Subscriber Data (RSD) request to the HLR.
In step 403, the HLR receives the RSD request, sends the ISD request to the MSC/VLR, including the identification of the MME to which the user belongs.
In step 404, the MSC/VLR records the subscriber data, and returns the ISD response to the HLR.
In step 405, the HLR returns the RSD response to the MSC/VLR.
In step 406, the MSC/VLR returns a roaming number to the HLR.
In step 407, the MSC/VLR continues the calling processing, and performs the paging after receiving the initial address message, wherein, the MME is positioned by using the MME identification obtained from the HLR.
But in the practical application, the HLR is likely not to return the MME address, and the most common reason is that some operators are unwilling to update for the HLR, in this way, the VLR cannot obtain the information necessary for paging, and the call will fail.